1. Field of Invention
This application relates generally to automatic test equipment and more specifically to measuring duty cycle with automatic test equipment.
2. Discussion of Related Art
In the manufacture of electronic components, such as semiconductor chips, there is often a need for measuring parameters of electrical signals. By comparing the measured parameters to expected values, components operating incorrectly can be detected. While a component is being designed, detecting incorrect operation can provide information to allow the design to be improved.
During manufacture, every component made is often tested at least once. Sometimes, semiconductor components are tested while still part of a wafer or at some intermediate stage of the manufacturing process. Components operating incorrectly at this intermediate stage might simply be discarded to save the cost of further processing. Other times, the results of testing are used to alter the manufacturing operation to reduce the number of defective components. For example, yield management software aggregates failures found in many components to identify manufacturing equipment that is out of calibration or other problems in the fabrication of the components. By altering the manufacturing process to remove these problems, the process yields a higher percentage of fully functioning components.
Results of testing can also be used to alter the manufacturing operation in other ways. For example, components operating incorrectly might be altered to operate satisfactorily, such as with laser trimming or built in calibration circuitry. Alternatively, testing might be used for “binning” the components. Components that do not perform as expected under certain test conditions might perform adequately under other, less stringent, conditions. For example, components that operate incorrectly at a temperature of 125° C. might perform adequately at 105° C. These components might be marked and packaged for sale at a lower temperature range. Likewise, components that do not operate correctly at high clock rates might meet all operational requirements at a lower clock rate. These components might be sold for operation at the lower clock rate. Assigning components maximum operating rates as the result of testing is sometimes called “speed binning.”
Automatic test equipment, sometimes called a tester, is designed to rapidly test semiconductor components. To economically test every component being manufactured, automatic test equipment must run a complete set of tests on a component in a short period of time, such as a few seconds. Automatic test equipment often includes a plurality of digital channels that each can either generate or measure a digital signal for one test point.
The tester runs “patterns.” A pattern is a program that causes the tester to perform a test or tests. The pattern contains a sequence of vectors. Each vector specifies the operation of all the digital channels during one cycle of the tester's operation. The tester executes the vectors in rapid succession to create the desired sequence of stimulus signals and measurements. The timing of the vectors can be controlled to set the speed at which a component under test operates during the test.
Each vector specifies for each channel whether, during a particular cycle, that channel is to generate a signal or measure a signal. Where the channel is to generate a signal, the vector specifies whether the signal should have a logical value of HI or LO. Conversely, when the channel is to measure a signal, the vector specifies the expected value of the signal. The channel outputs a fail signal if the measured signal does not have the expected value when measured.
A tester may also be programmed to control other operating parameters. For example, the voltage levels corresponding to a logical HI or a logical LO signal can usually be programmed. Further, the timing at which events occur within a cycle can be programmed. The time at which the channel should apply an output value can be programmed relative to the start of the cycle. Likewise, the time at which the channel should sample the signal to measure its value can also be programmed relative to the start of the cycle. The time at which a sample should be made is sometimes called the “strobe” time.
The tester includes failure processing circuitry that captures the fail signals generated by the channels. This information about failures is used to identify defective components or as an aid in diagnosing problems with the design of the component or the manufacturing operation used to make the component. One simple function that the failure processing circuitry can perform is to count the number of failures in each channel during a pattern.
The digital channels are designed for generating and measuring digital values. Traditionally, testing using digital channels indicates whether the device is outputting a logic HI or logic LO at a time when an output is expected. Testers often include “instruments” for generating or measuring analog signals. For example, an arbitrary waveform instrument generates an analog signal that has a waveform that can be programmed with almost any shape. Other instruments might rapidly sample an analog signal and perform advanced signal processing functions on the captured samples, such as to find a power spectral density or other characteristics of an analog signal. Yet other instruments might measure jitter in a signal.
One parameter of a signal that might be desirable to measure is the duty cycle of a clock signal. Traditionally, the duty cycle has been measured using bench-top instruments, such as oscilloscopes. Such measurements are not suitable for use in a manufacturing process where components must be quickly tested. Heretofore, the duty cycles of components have generally been “guaranteed by design,” meaning that the component was designed to produce a signal with a certain duty cycle, but each component manufactured was not tested to verify that it complied with the design.
We have recognized that this approach is likely to be less suitable for purchasers of semiconductor components as semiconductor components operate at higher speeds. Generally, the range of expected duty cycle for a properly functioning component is specified as a percentage of the clock period. As clock frequencies increase, the periods get smaller and the acceptable deviation in the duty cycle is smaller. With a smaller acceptable deviation, testing is more likely to be required to ensure all components manufactured meet the specification. Trimming, calibration or speed binning are more likely to be needed to provide components meeting the required specification. We have recognized that it would be desirable to provide a simple and fast way to measure duty cycles of components during their manufacture without requiring a special instrument.
Heretofore, some analog parameters have been measured without special instruments. The digital channels of the tester are sometimes programmed to make analog type measurements. One example is an “edge find” routine, sometimes called a “timing search.” “Edge find” identifies the time at which a signal transitions through a predefined voltage (i.e. an edge), such as a digital signal transitioning from one state to another.
To perform an edge find, the signal is applied to a channel in the tester. The applied signal must contain periodic copies of the edge. Periodic copies of the edge are inherently contained in a periodic signal, such as a clock. Where the signal is not inherently periodic, a periodic signal can be generated by repetitively generating the portion of the signal containing the edge. Where the edge find routine is performed by a tester, a component under test can be controlled to repetitively generate a portion of a signal by repetitively executing an entire test pattern or alternatively by looping through a subset of the test pattern.
As an example of an edge find measurement on one test vector, the digital channel receiving the signal is programmed to measure the value of the signal and to expect the value to be a logical LO. The channel is programmed to recognize as a LO signal any signal with a value below a threshold voltage. The threshold is set to be near the mid-range of the edge. When the value of the signal is above this threshold, the channel indicates the measurement “failed.” Conversely, the channel indicates a pass when the voltage of the signal is below the threshold.
This measurement provides information about the value of the signal at one strobe time. In an edge find routine, measurements are repeated for many strobe times. The strobe time is incremented for successive measurements, in a search for two strobe points with a specific time difference that report different results (e.g. one reports pass, and the other reports fail). The time difference between these strobe points is often described as the measurement resolution, because the signal transition is known to occur somewhere between these two points. Multiple search algorithms have been applied with a goal to find the transition point at a desired resolution with a minimum number of strobe points.
While such a technique is useful, there still exists a need for a way to quickly and at low cost measure the duty cycle of a signal.